1. Field of the Invention
The invention relates to a relative visibility measuring process and device.
2. Description of the Related Art
The requirement for fog measurement or visibility determination which is as error-free as possible results from various public regulations. Thus, for example, a minimum visibility in [m] corresponds to a prescribed speed limit in [h/h] of double its numerical value for road traffic in the Federal Republic of Germany. A special regulation in the StVO [Road Traffic Act] is that a maximum speed of 50 [h/h] must be observed in a visibility of less than 50 [m]. Takeoff and landing bans come into force at airfields if specific visibilities are not achieved. Exact and reliable test equipment is required to comply with and monitor these regulations.
Apparatuses for fog measurement on the basis of a transillumination process are known, which comprise a transmitter and a receiver which is arranged a short distance away from the transmitter (approximately 1 [m] to approximately the roadway width). In ideal visual conditions, the signals transmitted by the transmitter are detected by a specific signal amplitude. The received signals are attenuated by absorption and scatter in fog, rain, smoke etc. The signal attenuation becomes greater the thicker the fog or the like, that is to say the shorter the visibility. A fog warning is initiated if a specific signal threshold is not achieved.
Incorrect measurements in the form of unknown situations which limit the visibility, such as fog, for example, and fog warnings caused by them can result from the occurrence of locally limited "fog pockets" in the relatively short measurement path.
Since the transmitter and the receiver are mounted on a robust beam, which as a rule is metallic, the effect of such "fog pockets" can additionally be caused by the test equipment itself in unfavorable weather (for example, a rapid rise in the air temperature, particularly after a cold night) as a result of condensation of the fog droplets on the large cold mass of the beam.
On the other hand, incorrect measurements in the form of incorrect fog warnings, that is to say without any fog or the like occurring, can be caused by contamination of the outer surfaces of the transmitter and receiver since such contamination also leads to attenuation of the received signals and thus to the initiation of the false fog warning.
A false fog warning is likewise initiated if an obstruction comes between the transmitter and receiver (for example, a bird). The attenuation of the received signals caused by the obstruction likewise initiates a false fog warning.
It is not possible to differentiate between the above-mentioned measurement disturbances and the actual fog warnings using the transillumination process described.
Another process, which is disclosed in DE-AS 21 56 063, comprises different reflectors being positioned at different distances from a transmitting/receiving device. The reflection properties of these reflectors are selected such that the received signals from all the reflectors are of equal magnitude at a specific reference visibility. The received signals can be distinguished on the basis of their range-dependent propagation time. If the signal level which applies to the reference visibility is not achieved, the range of the associated reflector defines the maximum visibility.
The distances between the reflectors, which are located on a line, must be selected to be relatively large in order to be able to separate the received signals from one another reliably and to avoid over driving effects in the receiving device. The visibility determination is thus relatively coarse.
DE-AS 25 13 061 describes a development in which the reflectors which are erected at different intervals are laterally offset with respect to one another. The transmitting/receiving unit scans the reflectors successively so that the scan position defines the respectively illuminated reflector. The number of reflectors which are "visible" at a preselected signal amplitude defines the visibility. The measurement disturbances already mentioned for the transillumination process can also not be identified here.
In addition, apparatuses for fog measurement are known which operate using a scattered light process. They comprise a transmitter and a receiver which are arranged alongside one another. The receiver detects elements of the signals transmitted from the transmitter (generally light) only when there are back-scattering particles, for example fog or the like, in front of the transmitter/receiver arrangement. The magnitude of the back-scattered signal element depends on the number of fog droplets acting as scatter centers, that is to say the density of the fog, and thus on the visibility. If the received signals exceed a specific signal threshold, a fog warning is initiated. Since the amplitude of the received signals caused by backscatter are only small, the signal threshold must likewise be selected to be low. As a consequence, sufficiently large received signals and thus false warnings can also be initiated by fixed obstructions at a relatively long distance having a good reflection capability in the measurement beam path.
On the other hand, if fog is actually present, the back-scatter signals can be attenuated by contamination of the outer surfaces of the transmitter and receiver to such an extent that the received signals do not exceed the warning threshold. A required fog warning is thus not initiated.
Those apparatuses which operate using the scattered light process can thus also not distinguish between measurement disturbances and actual visibility limiting situations.